The broad objective of this research project is to understand the role of calcium ion as an intracellular messenger mediating hormone action on adipose tissue cells. Several studies have suggested that calcium acts at a step prior to formation of cyclic AMP while other studies have supported the view that calcium acts at a step antecedent to cyclic AMP formation. During the past year we have studied the effects on lipolysis and on cyclic AMP formation of several agents (Mn2 ion, Co2 ion, tetracaine La3 ion and D600) which interfer with transmembrane calcium movements. Of these, Mn2 ion, Co2 ion and tetracaine all inhibit lipolysis that had been activated with isoproterenol, ACTH or theophylline. However, cyclic AMP levels are always higher in the presence of these agents, indicating that they do not act by blocking formation of cyclic AMP. La3 ion and D600 do not affect lipolysis. We have suggested that Mn2 ion, Co2 ion and tetracaine act by entering the cell and therein interferring with an intracellular redistribution of calcium, which is essential for activation of lipolysis. La3 ion and D600 are thought not to penetrate cells and are therefore inactive. In another series of experiments we have studied the effects on lipolysis and cyclic AMP formation of the bivalent cation ionophores A23187 and X537A. Both X537A and A23187 decreased lipolysis in hamster adipocytes that had been activated with isoproterenol, theophylline and ACTR. However, these ionophores failed to lower cyclic AMP levels; indeed X537A actually increased cyclic AMP. Curiously, A23187 and X537A seemed to be inactive when tested against rat adipocytes. We suggest that these ionophores may be acting by a mechanism similar to that of alpha adrenergic agents and that an increase in intracellular calcium may be capable of inhibiting lipolysis.